In recent years, wireless data networks have made significant progress, with both the world-wide upgrade of cellular networks to support wide-area data access, and the widespread deployment of IEEE 802.11 based wireless local area networks (WLAN).
Those two technologies have significant differences. First of all, they provide different coverage: cellular networks can use large cells and through the support of mobility offer nation-wide service continuity, while wireless local area networks are limited to localized and relatively small hot spots.
Second, data rates offered from the two networks are different, with cellular networks supporting data rates relatively low, in the order of hundreds of kilobits, and wireless local area networks providing data rates significantly higher, reaching up to 54 Mbps in the particular case of IEEE 802.11a and IEEE 802.11g standards.
In particular, while cellular networks operate in infrastructure mode, with fixed base stations serving mobile users, WLAN can operate also in ad-hoc mode, where clients relay packets over multi-hop wireless links. All those differences make wireless LAN and cellular networks complementary technologies that can satisfy different necessities of the users.
For this reason, in the last few years a certain interest arose in the market for so called dual-mode mobile handsets, that provide both cellular (2G/3G) and wireless LAN connectivity. The availability of this kind of handsets offers the opportunity of combining the two technologies and leverages the advantages of each other.
It is therefore felt the need of efficiently integrating the above two technologies in a hybrid communication network enabling both cellular and WLAN connectivity.
An interesting solution for the integration of the cellular system with WLAN operating in ad-hoc mode is described in the paper “UCAN: A Unified Cellular and Ad-Hoc Network architecture” (authors: H. Luo, R. Ramjee, P. Sinha, L. Li, S. Lu), where mobile terminals with both 3G cellular link and IEEE 802.11-based peer-to-peer links are taken into account.
The 3G base station forwards packets for destination clients with poor channel quality to proxy clients with better channel quality. The proxy clients then use an ad-hoc network composed of other mobile terminals employing IEEE 802.11 wireless links to forward the packets to the appropriate destinations, thereby improving cell throughput.
The discovery of the proxy clients is accomplished through on-demand protocols that exploit the 3G infrastructure to reduce complexity and improve reliability. The solution also takes into account that mobile terminals acting as relays should be somehow rewarded for the energy consumption that they experience when they forward data of other users.
A secure crediting system is proposed, where proxy and relay clients are awarded credits that can be redeemed in the form of shared revenue, or increased priority in the future call admission, packet scheduling and/or network traffic engineering.
Moreover, in paper “Profit-Based Routing for Multihop Coverage Extension in Wireless Networks” (authors Y.-J. Choi, J. H. Jun and S. Bahk) relaying nodes receive a revenue R from the operator as a reward for their energy consumption.
Assuming that the energy consumption represents a cost Ci for the user i, the proposed scheme takes into account a path in the ad-hoc network only if the profit for all the terminals that act as relay is positive, i.e. if R>Ci for all i, and if the total cost experienced by the operator, that is the sum of all the revenues R paid to the relaying nodes, is lower than the revenue gained from the established connection.
Under this constraint, the profit-based routing scheme presented in the aforementioned work selects the path that allows reaching the base station with the minimum number of hops, so that the amount of revenues paid from the operator is as small as possible.
The assumption behind this solution is that the ad-hoc network will be used only when it is not possible to directly connect to the base station: when a direct connection is not available the shortest path that allows reaching a terminal in the base station coverage will be selected, given that the cost paid from both the operator and the relaying nodes is not greater than their revenue.